1. Field of the Invention
The invention relates to drilling bits in general and to combined pilot and reamer bits, in particular.
2. Prior Art
Torque is imparted to drill bits in one of two primary means. First, the entire drill string is turned from the surface. Second, “mud motors” (impeller driven shafts) are installed in the drill string near the bit. Drilling “mud” is pumped through the drill string, through the impeller, and out apertures on or near the pilot bit. The mud passing through the impeller turns the impeller which then turns the bit.
The drilling mud serves several other purposes as well. Once the mud flows out of the drill string near the bit, it is pumped back to the surface of the well. As it is flowing through the well bore, it serves to support the bore and hold back fluid from the surrounding formation. The mud also lubricates the bit or bits and entrains cuttings from the bit into the mud stream, carrying the cuttings back to the surface of the well.
Although the downhole mud motors are used to generate torque, higher torque can generally be imparted by turning the bit from the surface. Although torque from mud motors may be increased by linking mud motors in series, the torque which may be imparted at the surface is still generally greater. However, surface imparted torque has its limits. Too much torque and—rather than turning the bit—the drill string itself will twist in two.
Prior art drill systems are known to contain at least two types of drills: pilot drill bits and reamers. Pilot drill bits are located at the terminal end of the well bore. Reamers are generally located up string from the pilot drill bit. The pilot drill bit creates an initial well bore or “pilot hole.” A reamer increases the diameter of the bore.
One current reamer is known as a “bi-center bit.” Where a conventional reamer is a symmetrical pair or set of blades extending from a housing, the blade or blades of a bi-center bit extend only (or primarily) on one side of the housing. As the housing turns, the bi-center bit reams out the bore wall surrounding the housing. It can be seen that a bi-center bit with one four inch long reamer blade will increase the circumference of a well bore the same amount as a conventional symmetrical reamer with two oppositely aligned four inch long reamer blades; however the diameter of the bi-center bit will be four inches narrower than that of the conventional symmetrical bit. Thus, the bi-center bit will be able to fit down smaller casings than a conventional symmetrical reamer.
The pilot bit is often the limiting step in drilling. For example, in one recent drilling run, a 6½ inch diameter hole was drilled over a length of about 1400 feet using a polycrystalline diamond compact (PDC) pilot bit. The average rate of penetration (ROP) was about 23 feet per hour. After the pilot hole was drilled, the drill string was removed and the hole was reamed using 9⅞ inch reamer. The average ROP for the reamer over the same 1400 feet was about 154 feet per hour, almost seven times as fast as the pilot bit despite the fact that the reamer was cutting about 2.3 times the volume of earth as the pilot bit.
One reason the reamer is believed to be able to move so much faster than the pilot bit is because the pilot bit is believed to substantially weaken or “stress relieve” the formation in the area immediately surrounding the pilot hole. That is, when the pilot bit moves through a formation, the rock surrounding the path of the bit is shattered or at least substantially weakened by the passage of the pilot bit. Thus, when the reamer follows the pilot bit, it is able to travel much faster because the cutting is much less difficult.
The operation of drilling systems is expensive. Much of the equipment is rented by the day, and delays can substantially increase the cost of the well. As the previous example illustrates, to separately drill a pilot hole and then ream the bore out to the desired size requires the entire drill string to be removed and the well to be rerun. This can be very time consuming and thus very expensive. Accordingly, a drilling system that meets the following objectives is desired.